Paper coating compositions are used in paper industry to provide various physical properties to paper. Generally paper is coated with paper coatings to improve the printability, and to improve smoothness and gloss of the paper surface, and in particular to improve whiteness. Such paper coatings generally comprise an aqueous polymer dispersion, one or more pigments and other additives typically used in paper coating formulations. In addition to copolymers of styrene and butadiene and copolymers of styrene and acrylic acid ester, paper coating compositions with copolymers of vinyl acetate and ethylene (VAE) have become important.
During the offset printing process on coated paper, high levels of stress are experienced by the coating layer. This is a result of the increasing tack force developed by the ink layer as it loses its oil-based vehicle into the pore structure of the coating. When the tack force develops too quickly, or the coating strength is inadequate, the phenomenon of “dry pick” is observed, in which fragments of coating are removed by the ink. Additionally, in non-image areas of the print where aqueous fountain solution has been applied, the coating is weakened and subsequent printing with ink can cause removal of fragments of the coating, a phenomenon known as “wet pick”.
In consequence, in addition to the above mentioned basic qualities required in paper coatings, the paper coating must have high pick strength, to permit subsequent printing on the coated paper without “picking”. In particular, the paper coating must have sufficient wet pick strength. Known binders based on vinyl acetate ethylene copolymers do not show the desired wet pick strength.
U.S. Pat. No. 3,337,482 discloses a paper coating composition comprising a vinyl acetate ethylene copolymer with 2 to 30% of ethylene and 0 to 5% of an ethylenically unsaturated carboxylic acid. According to this patent, dry picking becomes too great if more than 30% of ethylene is copolymerized.
U.S. Pat. No. 3,642,680 discloses a paper coating composition with improved pick strength, which is obtained by polymerizing ethylene and vinyl acetate in the presence of an acrylic seed.
U.S. Pat. No. 3,404,112 describes a paper coating composition with good dry pick characteristics. An aqueous dispersion of a crosslinked vinyl acetate ethylene triallyl cyanurate terpolymer is used as binder.
In U.S. Pat. No. 3,812,072 vinyl acetate ethylene copolymer dispersions are used in paper coating compositions. Graft copolymers of vinyl ester on polyalkylene oxides are added to the paper coating compositions to improve compatibility of the vinyl acetate ethylene copolymer with further ingredients of the paper coating composition. Coating compositions comprising such graft copolymers had better dry pick strength.
High strength pigment binders for paper coatings are disclosed in U.S. Pat. No. 4,395,499. The latex binder comprises a polymer of vinyl ester, a polyethylenically unsaturated comonomer, an ethylenically unsaturated carboxylic acid, and optionally an alkyl acrylate. Picking is not discussed in this patent.
In U.S. Pat. No. 4,503,185 improvement of dry pick strength of a paper coating composition is achieved by copolymerizing vinyl acetate and ethylene with dicyclopentadienyl acrylate. Dry pick strength is drastically improved due to the crosslinking activity of the dicyclopentadlenyl groups, but wet pick strength remained unchanged.
In EP 0140227 A2 a pigmented paper coating composition is disclosed which comprises an aqueous polymer dispersion of vinyl acetate ethylene copolymers with copolymerized silane-functional comonomers and at least one comonomer selected from carboxyl-functional comonomers, amide-functional comonomers and polyethylenically unsaturated comonomers. These copolymers impart improved dry pick strength.
The subject of JP 06-220134 A2 is a paper/fiber treating, agent with excellent storage stability and flame retardancy, obtained by copolymerizing ethylene, vinyl chloride and vinyl acetate in the presence of protective colloid and a mix of anionic and nonionic emulsifiers.
EP 0432391 A2 concerns an emulsion binder for carpet comprising an emulsion polymer of vinyl ester, ethylene and a comonomer having a Tg of at least 50° C. The high Tg comonomer improves dry tuft binding properties of backed carpets.
EP 0381379 B1 discloses a process of offset lithographic printing of paper wherein the paper is coated with a coating composition comprising an emulsion of a copolymer of a lower vinyl ester, ethylene, and a vinyl ester of an alpha-branched monocarboxylic acid such as VeoVa 9®.
WO 2013/074902 A1 discloses blends of a vinyl acetate ethylene latex dispersion and a second latex dispersion with styrene butadiene copolymer or styrene acrylic acid ester copolymer. Paper coating compositions containing blends of this type exhibit higher dry pick strength than each of the components alone.
US 2012/0021237 A1 discloses an alkylphenol ethoxylate-free paper coating emulsion comprising a copolymer of vinyl acetate, ethylene, unsaturated carboxylic acid and polyethylenically unsaturated comonomer.
WO 2013/074902 teaches improvement of dry pick resistance of paper coatings based on silane-functional VAEs by copolymerizing ionic comonomers selected from unsaturated sulfonic acids/sulfonates and/or unsaturated phosphonic acids/phosphates.
It was therefore an object to provide an improved vinyl ester ethylene latex dispersion that imparts high wet pick strength into paper coating compositions.